Ability to add custom inline embeds/types/structured data to Richtext, beyond just formatting labels.
This solves a currently deal breaking use-case that Prismic can't achieve: Richtext with additional inline structured data. Eg: tooltips, footnotes, custom Links (that could for eg: open slideovers), inline media actions, etc.
Every other headless CMS I've used has some way of doing this. Contentful can embed entries/custom types inline, Sanity and Dato both have inline "blocks" (equivalent to slices). Even Wordpress had shortcodes.
Example of a recent project for a large client that required this — editors needed to be able to embed songs in articles, which played audio clips in a global player. This was done in Contentful by embedding inline "song" entries.